COLON-AEROGENES GROUP OF BACTERIA 273 



REVERSION OF REACTION 



The principle involved in the differentiation of the coli from 

 the aerogenes type of bacteria by means of the Clark and Lubs 

 media and the methyl red test is as follows. B. coli soon reaches 

 a growth-inhibiting or lethal zone of hydrogen ion concentration 

 and remains there; B. aerogenes, on the other hand, by virtue 

 of its peculiar metabolic properties, continues its action upon the 

 medium and progressively raises the pH value to a maximum 

 which does not in itself retard further development of the organ- 

 ism. B. coli rapidly attacks the glucose, with relatively large 

 acid production, while B. aerogenes produces more gas and corre- 

 spondingly less acid during the early stages of growth than the 

 coli type. B. aerogenes brings about a reversion of reaction which 

 after prolonged incubation becomes more and more apparent. 



This reversion of reaction in aerogenes cultures has led Kligler 

 and a number of other investigators to assume that it is due to 

 the production of ammonia from a peptone-glucose medium after 

 exhaustion of the sugar. In their further study of the subject, 

 Clark and Lubs disproved this view. They showed that reversion 

 of reaction can take place in a synthetic medium free from 

 peptone, and further proved that an extreme reversion can be 

 obtained in a synthetic medium containing an ammonium salt 

 as a source of nitrogen, although the content of total nitrogen is 

 reduced to a point at which its participation in any form in 

 changes of reaction of the medium would be insignificant. They 

 come to the conclusion, therefore, that "An increase in ammonia 

 may accompany the reversion of reaction, but the amount liber- 

 ated is inadequate to account for the extent of the reversion." 

 Under such conditions they believed that "It should not be as- 

 sumed that the reversion is due solely to ammonia production." 



These conclusions of Clark and Lubs do not give us a satis- 

 factory explanation as to the cause of the reversion, nor do they 

 lead us to think that they deny ammonia production in a glucose- 

 peptone medium. It remained for Ayers and Rupp (1918) to 

 furnish a plausible explanation for the phenomenon of reversion 

 in a sugar-peptone medium. They demonstrated that there is 



